The invention relates to a method for starting up a partial combustion or gasification process using a burner wherein finely divided carbon-containing fuel is supplied to a reactor or gasifier and an oxygen-containing gas is supplied separately from the said fuel to a reaction zone comprising a reactor or gasifier and is mixed with the said fuel adjacent the outlet of the burner in the reactor or gasifier. In particular, such gasification processes are used for preparing synthesis gas.
Synthesis gas, consisting mainly of carbon monoxide and hydrogen, is produced by partial combustion of finely divided fuel with a substoichiometric amount of a combustion medium like pure oxygen or an oxygen-containing gas such as air. Depending on the composition of the combustion medium the synthesis gas may also contain other substances, which may be useful or may be considered pollutants.
In gasification processes, usually fuel in a finely divided state is passed with a carrier gas to a reactor zone via a burner, while the combustion medium is either added to the fuel flow inside the burner or separately introduced into said reactor zone. Great care must be taken that the reactants are effectively mixed with each other. If the reactants are not brought into intimate contact with each other, the oxygen and fuel flow will follow at least partially independent paths inside the reactor. Since the reactor zone is filled with mainly hot carbon monoxide and hydrogen, the oxygen will rapidly react with these gases instead of with the fuel. The so formed very hot combustion products carbon dioxide and steam will also follow independent paths having poor contact with the relatively cold fuel flow. This behaviour of the oxygen will result in local hot spots in the reactor, thereby possibly causing damage to the reactor refractory lining and increased heat fluxes to the burner(s) applied.
Sufficient mixing of the fuel and the oxygen can be achieved by adding the oxygen to the fuel flow in the burner itself. A disadvantage of this method is that, in particular at high pressure gasification, the design and operation of the burner are highly critical. The reason for this is that the time elapsing between the moment of mixing and the moment the fuel/oxygen mixture enters into the reactor should be invariably shorter than the combustion induction time of the mixture, to prevent premature combustion inside the burner.
Moreover, the velocity of the mixture inside the burner should be higher than the flame propagation velocity in order to prevent flashback. However, the combustion induction time shortens and the flame propagation velocity increases at a rise in gasification pressure. Further, if the burner is operated at a low fuel load, the combustion induction time or flashback condition might easily be reached in the burner itself, resulting in overheating and possibly damage to the burner.
The problems of premature combustion in the burner itself or flashback will not occur if the fuel and the oxygen are mixed outside the burner in the reactor space itself.
In order to start the gasification process, a separate start-up (ignition) burner is used to ignite the gasification process. Usually, oil- or gasfired start-up burners are applied.
However, the handling of such separate start-up burners is complicated. It is therefore an object of the invention to provide a method for starting up a gasification process which makes the use of separate start-up burners (ignition burners) superfluous.
It is another object of the invention to provide a simpler, faster and safer operation of gasification processes than can be obtained by means of conventional separate start-up burners.